The present invention relates to a predictive coding system which performs high efficiency coding of image signals as of commercial television or television conference system through utilization of high correlation between adjacent picture elements in a picture frame and, more particularly, to a coding system with a noise shaping function by which noise resulting from predictive coding is shaped into noise which cannot easily be recognized visually.
With the existing standard television system, a picture formed by frames which are transmitted at a rate of 30 per second, and each frame has two continuous fields because of interlaced scanning every other scanning line. The elements which constitute the picture frame are called "picture elements"; since in the present specification digital processing is taken into account, one sample obtained by sampling will hereinafter be referred to as a picture element. Accordingly, in this instance the position of each picture element in the picture frame depends on the sampling frequency for digitizing signals.
To reduce the required number of transmission bits for transmitting information of each picture element, quantization is generally employed.
In the reduction of the required number of transmission bits by nonlinear or linear quantization, an increase in the quantization step size will decrease the number of bits but, at the same time, it will cause an increase in the amount of coding noise which is superimposed on coded image signals. Accordingly, in the case of obtaining a coded image as faithful to the original image as possible and of low coding noise, the quantization step size cannot be selected very large, and hence a substantial reduction of the required number of transmission bits is difficult.
To solve this problem, permit a sharp reduction of the required number of transmission bits and implement a high-quality coded image, the prior art has utilized a noise shaping filter in predictive coding.
However, conventional noise shaping predictive coding system using a noise shaping filter has a defect such as increase of quantization entropy.